Usually, when the rated current is 630 ampere or above in the case of power switches, electronic overload trigger devices or triggers are used. Thermal or thermomagnetic triggers, which use a bimetal or a bimetal strip, have not been suitable thus far for use in such high rated currents for various reasons.
While in the case of electronic triggers the current is measured electronically and the desired trigger time can be set, the trigger time in the case of thermal triggers is determined by the temperature at the bimetal and the temperature-dependent deflection of the bimetal. In the process, the bimetal is connected to the conductor at one or more point(s). The bimetal therefore heats up primarily at this/these connection point(s). This leads to the bimetal requiring a relatively long time to reach the deflection that is normally required for triggering. The trigger times that are usually required are therefore not achievable with bimetal triggers in the case of high currents. The self-protection of the power switch is not ensured by the excessively long trigger times either, especially in the case of many times the rated current, since excessively high I2t values can occur.
A further disadvantage of bimetal triggers is that the trigger precision in the case of high currents can be influenced by external influences such as a draught or the installation position of a power switch since the bimetal is normally attached in a freestanding manner in the case of power switches. This can lead to power switches with bimetal triggers triggering at different currents because of environmental influences. When measuring the trigger time, in particular in the case of many times the rated current, this becomes noticeable in varying trigger times.
U.S. Pat. No. 3,288,964 discloses a relay which comprises a U-shaped molded bimetal which is arranged substantially in parallel with a U-shaped molded portion of a current path of the relay.
U.S. Pat. No. 3,296,398 A discloses a safety switch having thermal overcurrent protection, in which a bimetal trigger is arranged substantially in parallel with a resistance heater and in a housing of the safety switch together with said heater.
US 2006/0232905 A1 also describes a thermal overcurrent protection device, in which a bimetal switch either is embedded in a conductor portion which has high electrical resistance and through which electric current flows or is sealed in together with resistance wires through which electric heating current flows.
U.S. Pat. No. 3,422,317 A relates to a thermal overcurrent protection device comprising bimetals which are U-shaped and arranged substantially in parallel with current paths. In order to compensate ambient influences, a separate U-shaped bimetal is provided which is exposed to the ambient temperature. An arm of this separate bimetal is coupled to arms of the bimetals, which are assigned to the current paths, via a lever, in order to compensate the influence of the ambient temperature on the trigger mechanism of the bimetals assigned to the current paths using the effect of the lever.